There's a thing where people will tend to hate those that almost agree with them more than they hate the ones that completely disagree with them. Is there a term for this? Has it been studied?
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The term (although not a single word) is the narcissism of small differences.
There's a Wikipedia article about it:
The narcissism of small differences (German: der Narzissmus der kleinen Differenzen) is the thesis that it is precisely communities with adjoining territories and close relationships that engage in constant feuds and mutual ridicule because of hypersensitivity to details of differentiation. The term was coined by Sigmund Freud in 1917, based on the earlier work of British anthropologist Ernest Crawley. In language differing only slightly from current psychoanalytic terminology, Crawley declared that each individual is separated from others by a taboo of personal isolation, a narcissism of minor differences.
Sectarianism is a form of bigotry, discrimination, or hatred arising from attaching relations of inferiority and superiority to differences between subdivisions within a group. Common examples are denominations of a religion, ethnic identity, class, or region for citizens of a state and factions of a political movement.
Maybe you could use the law of attraction to describe this phenomenon.
Like poles repel each other while unlike poles attract.
It may not have widespread acceptance but I find it conveys the message well enough (with visual imagery of like poles of a magnet repelling each other)